1. Field of the Invention
The present invention relates to projection apparatuses and in particular to miniaturized low-cost light projectors, as they are used for the representation of images, patterns, characters, or symbols or for the illumination of a photosensitive material.
2. Description of the Related Art
For the projection of images, either parallel methods, as it is the case for example with LCDs (liquid crystal displays) or with micro-mirror arrays, or scanning methods are used, as they are for example realized by a biaxially movable or two uniaxially movable mirrors. Disadvantageously, in the parallel methods, a comparably large substrate area in manufacturing and also a complex test procedure are required. Both result in a comparably high price for parallel-type projectors, so that methods working in parallel are not considered for a low-cost projection apparatus.
In the scanning methods, the projectors include one or two movable mirrors enabling deflection of a light beam about two deflection axes and two-dimensional deflection of the light beam, respectively. By the deflection, the light point generated by the light beam is moved on the image field, the intensity of the light beam meanwhile being modulated depending on the instantaneous projection place of the light point on the image field.
In order to enable a high degree of miniaturization with concurrently low production costs, micromechanically manufactured movable mirrors are employed in the scanning-type projectors. With the previous scanning-type systems, the projection is always based on a column and row representation of the image. In order to enable this column and row representation, in these systems the row frequency, i.e. the frequency of the deflection of the light beam or the light point in a horizontal direction, is small relative to the column frequency, i.e. the frequency of the deflection of the light beam along the vertical direction. The mutual ratio of these frequencies sets the amount of the resolvable rows and can only be increased by a so-called interlace method, in which at first all even-numbered rows and then all odd-numbered rows of an image are alternately scanned or represented.
In micromechanically manufactured scanning projectors or scanners, the achieving of low eigenfrequencies or resonance frequencies poses a fundamental problem, since the mechanical stability of the system decreases with the eigenfrequency. If the vertical deflection is to be excited in resonance or resonantly, the deflection mirror or the deflection mirrors thus have to be operated at a correspondingly even greater horizontal frequency. Alternatively, the vertical deflection has to be performed in the quasi-static operation, in order to be able to resonantly create a horizontal deflection. In the case of a resonant vertical row deflection of the light beam, a problem is that the horizontal column frequency has to be great relative to an anyway great resonant row frequency. The great horizontal deflection frequencies occurring therein induce dynamic deformation of the mirror plate, which leads to resolution problems in the projection. In the case of the quasi-static vertical row deflection, very high operating powers are required that make miniaturization of the control of the deflection unit or the mirrors impossible or the deflection unit very expensive. These problems also cannot be eliminated by a decrease of both frequencies, since the row frequency or the vertical frequency determines the image repetition frequency, and a too low image repetition frequency leads to a flickering of the image.
In Hagelin, P. M., Solgaard, O.: “Optical Raster-Scanning Displays Based on Surface Micromachined Polysilicon Mirrors” IEEE J. Selected Topics in Quantum Elecr., Volume 5, No. 1 (1999), pp. 67-74 as well as in the articles Hagelin, P. etc.: “Micromachined Mirrors in a Raster Scanning Display System”, Broadband Optical Networks and Technologies: an emerging reality. IEEE/LEOS summer topical meeting (1998), pp. 109-110, and Conant, R. etc.: “A raster-scanning fill motion video display using polysilicon micromachined mirrors”, Transducers +99, Int. Conf. Solid-State Sensors and Actuators, Sendai (1999), pp. 376-379, a projection apparatus is described that is based on the use of two electrostatically excited micromechanical movable mirrors. The low-frequency row deflection in a vertical direction is accomplished by a quasi-statically deflected mirror, while the high-frequency horizontal column deflection is obtained by a mirror excited in resonance. The frequency ratio is 6.2 kHz:20 Hz. The amount of the representable rows is limited to 310 by the frequency ratio. Corresponding to the row frequency, the image repetition rate is 20 Hz. It is disadvantageous in this projection apparatus that the image repetition rate is so low that the projected image is perceived as flickering. Moreover, the high-frequency operated mirror has dynamic deformations leading to significant resolution limitations, in particular at the image margin. The increase of the image repetition rate at a constant number of rows or an increase of the row number, as they could be achieved by an increase of the frequency of the high-frequency mirror, do not make sense due to the occurring dynamic deformations or lead to non-tolerable image flaws.
In Urey, H.; Wine, D.; T.; Osborn; “Optical Performance requirements for MEMS-scanner based microdisplays”, Proc. SPIE Volume 4178 (2000), pp. 176-185, and Wine, D. etc.: “Performance of a biaxial MEMS-based Scanner for Microdisplays Applications”, Proc. SPIE Vol. 4178 (2000), pp. 186-196, a projection apparatus is described that is based on the use of a biaxially suspended mirror. The low-frequency vertical deflection is conducted quasi-statically, while the high-frequency horizontal deflection is created resonantly. The vertical deflection frequency is about 55 Hz. The horizontal deflection frequency has been adjusted to 18 kHz in order to accomplish a row number of about 350. With this high horizontal deflection or column frequency, the dynamic deformation of the mirror plate is so great that, in particular at the margin area of the image, distinct resolution deteriorations occur.
In Schweizer, S. etc.: “Thermally actuated microprojector for optical display applications”, Proc. SPIE Vol. 4178 (2000), pp. 165-175, a projection apparatus is described that is based on the use of a mirror that may be excited to two vibrations at the same time, wherein the vibration axes are perpendicular to each other. The low-frequency vertical deflection is achieved by a quasi-static deflection of the mirror, while the high-frequency horizontal deflection is conducted in resonance. The image repetition rate is 50 Hz. The amount of the resolvable rows is limited to 100 rows by the vertical frequency of 5 kHz. In the margin area the image quality is lower than in the image center due to the dynamic deformation.